1. Field of the Invention
The present invention relates to a golf ball, and more particularly, to the golf ball which is not different in its flight performance depending on a different rotational axis thereof by improving the symmetrical property of dimples arranged thereon and which is improved in the aerodynamic lift and resistance thereof so as to fly the golf ball a long distance by appropriately varying the volume of dimples adjacent to each other.
2. Description of the Related Arts
Normally, 300 to 550 dimples are formed on the surface of a golf ball. Dimples are formed thereon to improve the aerodynamic characteristic thereof while the golf ball is flying so as to fly the golf ball a long distance.
When the golf ball is hit by a club, a backspin is imparted thereto. The rotational axis thereof serves as an important factor for increasing the flight distance thereof and flying it uniformly regardless of whether it is hit on the pole or the seam. It is not preferable for the golf ball to fly a varied distance according to a rotational axis thereof. That is, desirably, the flight distance obtained when it is hit on the seam is equal to that obtained when it is hit on the pole.
A golf ball approved by the golf association can be used in a title match. In the U.K. and the U.S.A., it is essential for golf balls to meet the requirements provided by Royal and Ancient and USGA, respectively. The symmetrical property test is one of the requirements. That is, it is examined whether or not there is a difference in the flight distance of a golf ball depending on a rotational axis, namely, between the seam hitting and the pole hitting.
A golf ball is formed by a split mold comprising upper and lower molds. Accordingly, a burr is formed on the surface thereof due to the seam between the upper and lower molds. It is desirable that the golf ball flies the same distance or in the same trajectory height irrespective of a hitting point, namely, a rotational axis. However, the trajectory height in the seam hitting tends to differ from that in the pole hitting due to the existence of the seam. The seam hitting means that the golf ball is hit in such a manner that the line connecting the poles serves as the rotational axis of the backspin thereof. The pole hitting means that the golf ball is hit in such a manner that a line perpendicular to the above-described rotational axis serves as the rotational axis of the backspin thereof. A golf ball having a great difference in flight distance thereof between the seam hitting and the pole hitting is not officially admitted. In order for a golf ball to be recognized as being superior, it is absolutely necessary that the difference in the flight distance thereof between the seam hitting and the pole hitting is less than the reference value officially recognized. That is, the golf ball is required to have the same flight performance in the seam hitting and the pole hitting and in addition, the same flight distance and trajectory irrespective of a manufacturing method.
As described above, the golf ball is required to have uniformity, namely, symmetrical property in its flight performance. But the following conventional dimple arranging methods are intended to improve the flight performance of the golf ball rather than the symmetrical property thereof: regular icosahedral arrangement shown in FIG. 8 and disclosed in Japanese Patent Publication No. 58-50744, modification of regular icosahedral arrangement of Japanese Patent Laid-Open Publication No. 2-45074, regular dodecahedral arrangement shown in FIG. 9 and disclosed in Japanese Patent Publication No. 57-22595, icosahedral-dodecahedral arrangement shown in FIG. 10 and disclosed in Japanese Patent Laid-Open Publication No. 60-234674, concentric arrangement shown in FIG. 11 and disclosed in Japanese Patent Laid-Open Publication No. 53-115330, and cubic octahedral arrangement shown in FIG. 12 and disclosed in Japanese utility Model Laid-Open Publication No. 63-186469, and Japanese Patent Laid-Open Publication No. 1-221182. Golf balls according to the above proposals have all a plurality of axes of symmetry on the surface thereof, but none of them are at right angles with each other. In addition, dimples are not symmetrical with respect to any one of the axes of symmetry.
In addition to the dimple arranging methods shown in FIG. 8 through 12, the regular octahedral arrangement as shown in FIG. 13 is a fundamental dimple arranging method and has been conventionally adopted since the time when a dimple was developed. According to this dimple arrangement, three axes of symmetry SL1, SL2, and SL3 are at right angles with each other and all dimples D are identical to each other in diameter, depth, and volume. Dimples are arranged symmetrically with respect to each axis of symmetry, thus having a preferable symmetrical property. Therefore, the regular octahedral arrangement is still the main current of dimple arranging methods.
The golf ball having dimples D shown in FIG. 13 formed thereon is preferable in its symmetrical property, however, has a disadvantage in respect of its flight distance. That is, as described previously, the golf ball flies with a backspin imparted thereto when it is hit. In order to fly the golf ball higher, it is required to set the separation point between air and the upper surface of the golf ball as backward as possible compared with the separation point between air and the lower surface of the golf ball so as to make air pressure existing above the golf ball smaller than that existing below the golf ball. The separation of air, from the golf ball, existing above the golf ball can be accelerated by making the air in the periphery thereof turbulent. In order to make air in the periphery of the golf ball turbulent, it is necessary to arrange dimples irregularly on the surface thereof while the symmetrical property and uniformity thereof are maintained in consideration of a favorable balance between the aerodynamic lift and drag brought about by the aerodynamic effect of a dimple.
In view of the above viewpoint, the dimple arrangement as shown in FIG. 13 has a problem in increasing the flight distance of a golf ball because dimples are identical to each other in diameters, depths, and volumes, i.e., dimples are arranged so regularly that air in the periphery of the golf ball does not become turbulent.
Many proposals for forming dimples of different diameters have been made to generate a turbulent air flow in the periphery of a golf ball with a view to increasing the flight distance thereof as disclosed in Japanese Patent Laid-Open Publication No. 60-234674. The golf ball can fly a long distance indeed, but the diameter of the golf ball is ununiform depending on an axis thereof due to diameter-differentiated dimples. Therefore, a line adjustment from a putting point to the hole is difficult and the golf ball does not roll straight depending on a rotational axis.